Heretofore, a liquid level detecting apparatus which detects the liquid surface level of a liquid within a tank has been, for example, one comprising an ultrasonic sensor which is installed in the fuel, and a reflector plate which is installed in the fuel and which turns an ultrasonic wave from the ultrasonic sensor toward the liquid level, wherein the ultrasonic wave reflected by the liquid level is received by the ultrasonic sensor through the reflector plate so as to calculate the liquid surface level (refer to, for example, JP11-153471A).
In the liquid level detecting apparatus stated above, a cylinder having a square internal cross section is fixed to a bottom surface within the fuel tank. The ultrasonic sensor is attached to one end side of the cylinder so as to be capable of generating the ultrasonic wave toward the other end side of the cylinder, while the reflector plate which turns the ultrasonic wave having proceeded inside the cylinder, toward the liquid level is disposed on the other end side of the cylinder. The cylinder forms an ultrasonic transmission route in the liquid level detecting apparatus.
In the prior-art liquid level detecting apparatus stated above, the cylinder is formed having the square internal cross section, and a uniform cross-sectional shape in the direction of the longitudinal axis of this cylinder. That is, among four wall surfaces which constitute the cylinder, two opposing ones are in a relationship in which they are parallel to each other.
The ultrasonic wave proceeds inside the cylinder while iterating reflections by the respective wall surfaces of the cylinder. Herein, each time the ultrasonic wave is reflected by the wall surface, it undergoes partial transmission through the wall surface, etc., so that the energy of the ultrasonic wave decreases gradually.
In the cylinder which is included in the prior-art liquid level detecting apparatus stated above, that is, in the cylinder whose cross-sectional shape is uniform in substantially the whole region thereof in the axial direction, the intensity of the ultrasonic wave entering the reflector plate, in other words, the acoustic pressure level of the ultrasonic wave per unit area in the cross section of the cylinder becomes lower than the acoustic pressure level of the ultrasonic wave in the vicinity of the ultrasonic sensor.
Therefore, the acoustic pressure level of the ultrasonic wave which is turned by the reflector plate toward the liquid level lowers. Consequently, the acoustic pressure level of the ultrasonic wave which is reflected by the liquid level lowers. Accordingly, it becomes difficult to detect the liquid level at a high accuracy.
Moreover, in a case where the fuel has quaked on account of the vibration of a vehicle, the travel thereof on a sloping road, or the like, the liquid level within the fuel tank comes above the cylinder and becomes slant especially when it is high. On this occasion, it is sometimes the case that the ultrasonic wave having proceeded inside the cylinder and arrived at the liquid level is reflected by this liquid level to proceed outside the cylinder, and that the reflected ultrasonic wave is not received by the ultrasonic sensor. Then, the problem occurs that the indicated value of a fuel indicator becomes unstable.
Further, in consideration of the fact that the velocity of an ultrasonic wave in a liquid changes depending upon the temperature, pressure, etc. of the liquid, there has been proposed a construction wherein an ultrasonic sensor is attached to the outside surface of the bottom of a liquid container so as to be capable of generating the ultrasonic wave toward the upper part of the interior of the container, and an ultrasonic reflector which turns the ultrasonic wave from the ultrasonic sensor, toward this ultrasonic sensor is installed at the position of a predetermined measurement reference height at the lower part of the interior of the container (refer to, for example, JP2001-208595A).
According to the construction, the ultrasonic sensor detects two sorts of data, namely, the round-trip time between the ultrasonic sensor and a liquid level and the round-trip time between the ultrasonic sensor and the reflector. Since the distance between the ultrasonic sensor and the reflector is known beforehand, the velocity of the ultrasonic wave in temperature and pressure conditions at that point of time can be directly and accurately calculated from the round-trip time between the ultrasonic sensor and the reflector. The position of the liquid level can be accurately detected from the ultrasonic velocity thus obtained, and the round-trip time between the ultrasonic sensor and the liquid level.
In the prior-art liquid level detecting apparatus stated above, however, the reflector is a component separate from the ultrasonic sensor, and it becomes difficult to keep the positional relationship of both the constituents highly accurate, because of the construction in which these constituents are fixed to the container. The accuracy of the positional relationship between the ultrasonic generation sensor and the ultrasonic reflector is greatly relevant to the measurement accuracy of the ultrasonic velocity in the liquid. Therefore, when the accuracy of the positional relationship between both the constituents lowers, the ultrasonic velocity in the liquid cannot be detected at a high accuracy, making it impossible to accurately detect the liquid level position.